Among studies for enhancing the inter-layer strength of a composite laminated structure, or connecting between two composite laminated members, z-pinning technology is a relatively recently started field. Conventional studies that have been known for z-pinning can be summarized as follows.
The initial concept of z-pinning may be seen from U.S. Pat. No. 4,808,461 issued on Feb. 28, 1989 (hereinafter, “prior art 1”). The gist of the prior art 1 is to insert pins into the inside of laminated plates while the preform is collapsed by high temperature and high pressure during a curing process by placing and curing a so-called “reinforcing structure,” in which pins are inserted into a polyvinyl-series form in the thickness direction, a thin stainless plate is attached on the top, and a release film is attached on the bottom, on the top of a composite material for lamination at the time of curing (refer to FIG. 1). When the curing is completed, the residue of the collapsed and compressed preform is removed and the protruded pins are broken by physical force to be removed as much as the thickness, and thus the performance of inter-layer separation will be enhanced because pins are inserted and remained within the laminated structure in a direction of the laminated thickness. Though the prior art 1 is characterized by such a method and the above-mentioned “reinforcing structure,” it has not yet been known that the concept of the invention has lately been put to practical use.
After that, in U.S. Pat. No. 5,186,776 issued on Feb. 16, 1993 (hereinafter, “prior art 2”), there is disclosed a mechanical device and a method thereof capable of a series of operations in which pins are directly cured from a composite reinforcement fiber bundle, and ultrasonic vibration is added to the cured pins to be inserted into the composite structure, and furthermore, in U.S. Pat. No. 5,589,015 issued on Dec. 31, 1996 (hereinafter, “prior art 3”), there is disclosed a method and a system in which pins are inserted into a composite structure in a pre- or post-cured state by compressing pins to the pre-inserted compressible “form body” while adding vibration using an ultrasonic device. It is known that the prior art 3 has been commercialized for practical use to be utilized in a connecting joint of the afterbody duct structure of F/A-18 E/F. Thereafter, there have been U.S. Pat. Nos. 5,667,859 (1997), 5,800,672 (1998), 6,190,602 (2001), 6,291,049 (2001), 6,405,417 (2002), 6,436,507 (2002), or the like, and the initial concept using “reinforcing structure” or “form body” is continuously maintained.
FIG. 1 is a view illustrating a general method of curing a composite. On a cur 111 for curing a composite, a release agent 112 is applied, and composite laminated prepregs 121 is placed, and again a release agent 112 and a bleeder 113 for absorbing excessive resin is laid, and all the above are covered with a film back 115 attached by an adaptor 116 for providing vacuum, and then the fringe is sealed with a sealant 117. There are instances where a cork dam 114 may not be used according to a curing process for curing a composite laminated structure 121.
FIG. 2, as a view illustrating a conventional method of z-pinning, shows a concept of the prior art 1. When placing and curing a “reinforcing structure 230” as illustrated in FIG. 2 in which pins 231 are inserted into a compressible form 232, a stainless plate is attached on the top, and a release film is attached on the bottom, pins 231 are inserted into the inside of composite laminated prepregs 221 while a form 232 is collapsed by curing pressure and curing temperature.
FIG. 3, as another view illustrating a conventional method of z-pinning, shows a case where a composite structure having an irregular thickness is cured using a conventional z-pinning method. When the composite structure having an irregular thickness is cured by a conventional z-pinning method, it gets into trouble because a stainless plate on the top of a “reinforcing structure 330” may prevent the transformation. Even though a “form body” which is not attached with a stainless plate is used, when a “form body (similar to 330, 230)” having a regular length of pins 331 is used, it is difficult to satisfy all the conditions because the composite may include a thick portion 322 and a thin portion 323 in its thickness. In other words, if the length of pins 331 is adjusted to the thick portion 322, it may not satisfy a fundamental requirement of composite curing (during a curing step, it is required that a high pressure of 7-8 atm or about 20 atm according to the material should be applied to a composite laminated prepregs in order to remove bubbles and voids created inside the composite laminated structure and discharge excessive resin) because the length of pins 331 is long for the thin portion 323 and thus curing pressure may not be applied to the composite laminated prepregs 323. Furthermore, if the length of pins 331 is adjusted to the thin portion 323, there occurs a phenomenon that pins are inserted into only a part of the laminated structure in a direction of the thickness because the length of pins 331 is short for the thick portion 322, and thus an effect of z-pinning cannot be sufficiently achieved. Accordingly, in order to cure a composite structure having an irregular thickness using a conventional method of z-pinning, it should be made by changing a thickness of the “form body 330” based on a thickness of the laminated structure, but this process may not be easily achieved due to a characteristic of the structure of compressible form 322.
As described above, according to conventional prior arts, a so-called “form body” (“reinforcing structure” may be seen as a kind of “form body”) having a characteristic that pins are inserted in advance in a form to insert the pins into the composite laminated structure, and pins are inserted into the inside of the composite laminated structure while the form being compressed and broken by outside vibration load or curing pressure, is used as a mediator, and this “form body” is a feature of the technology. However, in a case where the “form body” is utilized using such a conventional technology, material waste and process inefficiency cannot be avoided because the compressed and broken form should be artificially removed after pins are inserted, and also the protruded portion of pins should be forcibly fractured and removed.
Moreover, a method of inserting pins into a composite laminated structure using such a form body or reinforcing structure has a great problem that inter-layer strength becomes irregular according to its position in a composite laminated structure having an irregular thickness, which has been produced by the prior art, because the inserted depth of pins is varied according to the thickness in a case where the thickness of the composite laminated structure is irregular. In addition, a conventional method of z-pinning to a composite laminated structure having an irregular thickness has also a problem that it cannot be easily applied since the construction of the equipment is complicated and the process is very difficult to achieve.